The present disclosure relates to vehicles, such as space vehicles, spacecraft and the like, and more particularly to a method and system for controlling an attitude and angular velocity of the vehicle.
A major challenge for vehicle designers is “pointing performance,” that is making sure that the payload, for example an antenna, is aimed at the right spot or target on the Earth or elsewhere. Pointing is often autonomous, meaning directed by onboard computers, as opposed to having a team on the ground constantly commanding the vehicle.
There are two major components affecting spacecraft autonomous pointing performance. One component is knowledge consisting of orbit determination and attitude determination. Orbit determination is knowing where the satellite is in its orbit, and what the orbit is or nature of the orbit, for example geostationary or some other type of orbit. Attitude determination is knowing the satellite's orientation with respect to some reference frame. The other major component is control. Control includes ways to change the satellite's orbit or attitude.
In addition to other sensors employed on a vehicle to determine the vehicle's orbit and attitude and to control the orbit, attitude and angular velocity, a satellite typically includes at least two inertial reference units (IRUs) for redundancy in case one fails. The IRUs measure or determine the angular velocity about each of the three orthogonal axes. Angular velocity may be defined as the spin rate of the vehicle about each one of the three axes. The need for a spare IRU has often been questioned. IRUs typically include three gyroscopes or simply gyros. Each gyro measures angular velocity about a different axis. Some IRUs are internally redundant. They include four gyros. In the event of a failure of any single gyro, the IRU software may be reprogrammed to use the remaining three gyros. Accordingly, there is some basis for maintaining that a vehicle with an internally redundant IRU should not need a separate backup IRU. This would save the expense of the spare IRU and reduce the weight of the vehicle which can translate into a significant fuel savings in launching and placing the vehicle in orbit.
Unfortunately, while failure of a gyro within an IRU is apparent from telemetry, the IRU does not provide enough information to easily decide, which one of the gyros has failed. Therefore, a ground team must determine which of the four gyros failed, then reconfigure the IRU for use with the remaining three gyros. This can take several hours which cannot be afforded in critical situations. Therefore, a separate IRU is often required for backup.